This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-119808, filed Apr. 20, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a color cathode ray tube, particularly, to a color cathode ray tube capable of suppressing the fluctuation in the cathode current brought about by the thermal expansion taking place in the members constituting the electron gun assembly.
In general, a color cathode ray tube comprises an envelope including a panel having a phosphor screen formed therein, and a funnel integrally bonded to the panel and including a neck. An electron gun assembly is received in the neck of the funnel.
A so-called xe2x80x9cin-line-typexe2x80x9d electron gun assembly, in which three electron guns are arranged in a line, is mainly used nowadays as the electron gun assembly of a color cathode ray tube. The in-line-type electron gun assembly comprises in general a beam generating section called triode, which includes a cathode electrode, a first electrode, and a second electrode, and a main lens section for focusing the three electron beams on a phosphor screen. The electron gun assembly is received in the neck of cylindrical structure having a diameter of about 20 to 40 mm, and a stem section in the shape of a circular glass is welded to the neck. Stem pins made of a conductive metal are buried in the stem section such that the electron gun assembly within the tube is connected to the circuit outside the tube via the stem pins, and the electron gun assembly within the tube is held by the stem pins in the stem section so as to be fixed within the tube.
In the triode section referred to above, voltage of hundred and scores of V is applied from outside the tube to the cathode electrode through the stem pins. Also, 0 V and hundreds of V are applied to the first and second electrodes, respectively, from outside the tube through the stem pins. The cathode electrode is provided with a heater for generating electrons from the cathode electrode. By heating the cathode electrode by the heater, an electron beam is emitted from the cathode electrode. The electron beam emitted from the cathode electrode passes through the beam-passing apertures of the first and second electrodes so as to be guided to the main lens section and, then, finally focused by the main lens section on the phosphor screen.
The main lens section is formed of at least two electrodes including a final accelerating electrode connected to an anode to which is applied a high voltage of about 25 to 30 kV through an inner conductive film coated on the inner surface of the neck section, and a focus electrode to which is applied a voltage about 20 to 40% of the anode high voltage through the stem pins.
In general, the final accelerating electrode and the focus electrode are arranged to face each other such that the beam-passing holes of these two electrodes are positioned apart from each other by about 1 mm. Applying a potential difference between these mutually facing two electrodes forms the main lens section, and the electron beam is focused on the phosphor screen by the main lens thus formed.
Each of these electrodes is fixed to and supported by an insulating supporting bar made of, for example, a glass. To be more specific, the strap mounted to each electrode is buried in the insulating supporting bar so as to have each electrode fixed and supported.
As described above, a heater mounted inside the cathode electrode heats the cathode electrode. Originally, the heater is intended to heat the cathode electrode alone. However, each of the electrodes of the electrode gun assembly including the cathode electrode and the heater is fixed to and supported by an insulating supporting bar made of, for example, a glass. As a result, the heat generated from the heater is transmitted to not only the cathode electrode but also to the other electrodes of the electron gun assembly by the heat conduction via the insulating supporting bar. These electrodes are also heated by the heat radiated directly from the cathode electrode itself so as to lead to the temperature elevation.
The temperature elevation caused by the heat of the heater is most prominent in the first electrode positioned closest to the cathode electrode. Then, the temperature elevation is gradually lowered in the second electrode, the third electrode, et seq. as the distance from the cathode electrode is increased.
What is serious is the temperature elevation of the first and second electrodes serving to control the generation limit (cutoff) of the electron beam from the cathode electrode. If the temperature of these electrodes is elevated, these electrodes are thermally expanded so as to change the distance between these electrodes and, thus, to change the cutoff. As a result, the cathode electrode current is changed with increase in the temperature elevation of the electrodes.
It should be noted that the first electrode and the second electrode are formed of relatively thin plates in many cases. Therefore, if the first electrode and the second electrode are thermally expanded and deformed, the distance between these first and second electrodes is changed so as to change the cutoff. As a result, the cathode electrode current is changed in accordance with elevation of the electrode temperature.
It is possible for the problem described above to take place also in the third electrode the electric field of which somewhat affects the cathode electrode. However, the effect of the electric field given from the third electrode to the cathode electrode is markedly smaller than that given from any of the first and second electrodes to the cathode electrode. In addition, the temperature elevation of the third electrode is small because the third electrode is positioned remote from the cathode electrode. It follows that the particular problem is substantially negligible when it comes to the third electrode.
It may be possible to overcome the above-noted problem by using materials low in thermal expansion coefficient for forming the first electrode, the second electrode, etc. However, it is impossible to suppress the thermal expansion to zero and, thus, it is necessary to design the first electrode and the second electrode based on a subtle combination of the thermal expansion coefficients.
It should also be noted that a wire made of a conductive material is welded to each electrode for the electrical connection to the circuit outside the tube. When it comes to the thin plate-like electrodes such as the first electrode and the second electrode, the electrode is provided with a welding margin to which one end of the wire for the electrical connection to the circuit outside the tube is welded, and the other end of the wire is welded to the stem pin.
Naturally, the wire welded to the electrode is also heated by the heat of the heater so as to be thermally expanded. What should be noted is that, since the wire is welded to a part of the electrode, the thermally expanded wire pushes the welded part of the electrode, giving rise to a problem that the distance between the adjacent electrodes is changed in the vicinity of only that portion of the electrode to which the wire was welded. It follows that, even if the materials of the first electrode and the second electrode are selected exquisitely, the first electrode or the second electrode is locally deformed.
The local deformation brings about a change in the cathode electrode current in only one of the three cathode electrodes arranged in-line so as to disturb the current balance of the three electron beams in accordance with temperature elevation of the wire. As a result, the color of the image displayed on the phosphor screen is prominently changed.
In order to overcome the problem, it is necessary to improve the supporting strength of the electrodes of each electron gun assembly, to improve the shape of the strap of each electrode buried in the insulating substrate, and to improve the shape, material and thickness of the electrode for increasing the flexible strength of the electrode. However, it is difficult to achieve a sufficient reinforcement in the electron beam generating section in which a plurality of electrodes are arranged with a relatively small clearance provided therebetween.
As described above, the prior art is defective in that the wire welded to each of the first electrode and the second electrode of the electron gun assembly for the electrical connection to the circuit outside the cathode ray tube is thermally expanded so as to push the first electrode or a part of the second electrode, thereby changing the distance between these electrodes. What should be noted is that the cathode electrode current is changed by the thermal expansion of the wire so as to change the color of the displayed image.
Also, it is difficult to take sufficient measures against the problem because the distance between the first electrode and the second electrode is small.
An object of the present invention is to provide a color cathode ray tube capable of suppressing the change in the cathode electrode current and, thus, capable of displaying a satisfactory image low in the color change, even if the conductive material connected to the electrode of the electron gun assembly for electrical connection to a circuit outside the tube is thermally expanded.
According to a first aspect of the present invention, there is provided a color cathode ray tube, comprising:
an envelope including a panel having a phosphor screen mounted therein, a funnel contiguous to the panel and including a neck;
an electron gun assembly including three cathode electrodes received in the neck at an edge portion of the funnel and heated by heaters sealed in the neck, electrodes having apertures corresponding to the three cathode electrodes and forming a section, together with the cathode electrodes, for generating electron beams, electrodes for forming a main lens section for focusing the electron beams on the phosphor screen, and an insulating supporting bar for supporting the electrodes;
a stem section having a portion welded to the neck section and stem pins buried in the circular portion for holding the electron gun assembly and for electrically connecting each of the electrodes to the circuit outside the tube; and
a conductive member having one end connected to the stem pin and the other end connected at least one electrode included in the electron gun assembly, wherein the conductive member includes a folded buffering section for buffering the thermal expansion force accompanying the heat generation from the heater.
It should be noted that, in the color cathode ray tube of the present invention, a folded buffering section for buffering the thermal expansion force accompanying the heat generation from the heater is arranged in the conductive member connected to the electrode, for electrical connection to the circuit outside the tube. The folded buffering section is formed between that portion of the conductive member which is connected to the electrode and that portion of the conductive member which is connected to the stem pin. It follows that, even if the heat generated from the heater buried in the cathode electrode thermally expands the conductive member, the thermal expansion force of the conductive member is buffered by the folded buffering section so as to weaken the force in the direction of pushing the electrode. As a result, a change in the distance between adjacent electrodes, which is caused by the thermal expansion of the conductive member, can be eliminated so as to suppress the fluctuation in the cathode current. It follows that it is possible to provide a color cathode ray tube capable of displaying a satisfactory image free from the color change.
The present invention also provides a color cathode ray tube in which the conductive member is connected at one end to the stem pin and also connected at the other end to the electrode positioned closest to the cathode electrode included in the electron gun assembly. In the color cathode ray tube of the particular construction, the force in the direction of pushing the electrode positioned closest to the cathode electrode is weakened so as to eliminate the change in the distance between the adjacent electrodes, which is caused by the thermal expansion of the conductive member, thereby suppressing the change in the fluctuation of the cathode current. It follows that the color cathode ray tube of the present invention is enabled to display a satisfactory image free from the color change.
Further, the present invention provides a color cathode ray tube, in which the conductive member is connected at one end to the stem pin and also connected at the other end to each of the first electrode positioned closest to the cathode electrode and the second electrode positioned adjacent to the first electrode. In the color cathode ray tube of the particular construction, the force in the direction of pushing the first electrode positioned closest to the cathode electrode and the second electrode positioned adjacent to the first electrode is weakened so as to eliminate the change in the distance between the first and second electrodes, which is caused by the thermal expansion of the conductive member. As a result, it is possible to suppress the fluctuation of the cathode current so as to enable the color cathode ray tube to display a satisfactory image free from the color change.
Further, the present invention provides a color cathode ray tube, in which the folded buffering section of the conductive member is folded away from the inner wall of the tube in the neck. In the color cathode ray tube of the particular construction, the inner wall, which is an insulator, of the neck is charged up by the positive charge of the anode high voltage, making it possible to prevent the spark generation and the glow discharge taking place between the inner wall of the neck and the conductive member because of the approach of the conductive member toward the inner wall of the neck. It is also possible to prevent the short circuit with another conductive member.
Further, the present invention provides a color cathode ray tube, in which the folded buffering section of the conductive member is in the shape of a crank having at least one folded portion folded at an angle falling within a range of between 45xc2x0 and 135xc2x0 relative to the axial direction of the tube in the neck. In the color cathode ray tube of the particular construction, the thermal expansion force of the conductive member is resolved in the folded portion of the folded buffering section into the force in the axial direction of the tube and the force perpendicular to the axial direction of the tube. What should also be noted is that, since the folding angle falls within a range of between 45xc2x0 and 135xc2x0, the force in the axial direction of the tube is smaller than the force in a direction perpendicular to the axial direction of the tube. In other words, the force in the pushing direction of the electrode included in the electron gun assembly, i.e., the force in the axial direction of the tube, is weakened so as to eliminate the change in the distance between the adjacent electrodes caused by the thermal expansion of the conductive member. It follows that it is possible to suppress the fluctuation of the cathode current so as to enable the color cathode ray tube to display a satisfactory image free from the color change.
It is undesirable for the folding angle of the folded buffering section to be smaller than 45xc2x0 or to be larger than 135xc2x0 because the force component in the axial direction of the tube is rendered large in this case so as to prevent the folded buffering section from performing its proper function.
Further, the present invention provides a color cathode ray tube, in which the folded buffering section of the conductive member is formed in the shape of a continuous wavy curve having a folding angle falling within a range of between 45xc2x0 and 135xc2x0 relative to the axial direction of the neck.
Further, the present invention provides a color cathode ray tube, in which the folded buffering section of the conductive member is formed in the shape of saw teeth having a folding angle falling within a range of between 45xc2x0 and 135xc2x0 relative to the axial direction of the neck.
Still further, the present invention provides a color cathode ray tube, in which the folded buffering section of the conductive member is formed in the shape of a semi-circular curve having a folding angle falling within a range of between 45xc2x0 and 135xc2x0 relative to the axial direction of the neck.
In the color cathode ray tube of the particular construction, the conductive member including the folded buffering section in the shape of a continuous wavy curve, in the shape of saw teeth, or in the shape of a semi-circular curve permits weakening the thermal expansion force of the conductive member in the axial direction of the tube. To be more specific, the force component of the thermal expansion force for pushing the electrode included in the electron gun assembly, i.e., the force component in the axial direction of the tube, is weakened, compared with the force component in a direction perpendicular to the axial direction of the tube. As a result, it is possible to suppress the change in the distance between the adjacent electrodes caused by the thermal expansion of the conductive member. It follows that it is possible to suppress the fluctuation of the cathode current so as to enable the color cathode ray tube to display a satisfactory image free from the color change.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.